1. Field of the Invention
This invention relates to a process wherein a fluid stream comprising a noxious gas, e.g., nitric oxide (NO), and sulfur dioxide (SO.sub.2) is treated with a water-soluble polymeric chelate of a polyvalent metal to adsorb the nitric oxide. Water and the imidodisulfonate subsequently formed are separated from the polymeric chelate by means such as dialysis or ultrafiltration. The polyvalent metal chelate is then recycled and re-used. More specifically, the present invention relates to a process of removing NO from a gas stream using an aqueous solution of a water-soluble polymeric chelate of iron in the Fe (II) state, separating the imidosulfonate and a portion of the water by concentrating the aqueous water-soluble polymeric chelate of iron using ultrafiltration or dialysis, and recycling and re-using the polymeric chelate of iron (II).
2. Relevant Art
R. H. Hass in U.S. Pat. No. 4,448,899 discloses a process for removing NO from a gas stream by absorption with a water-soluble iron (II) chelate. The SO.sub.x and NO pollutants dissolved in the absorbent during regeneration are converted to hydrogen sulfide and nitrogen, respectively.
J. O. Porath in U.S. Pat. No. 4,423,158 discloses introducing a monomeric chelate-forming group into a polymer such as polystyrene. There is obtained an adsorbant for bivalent or multivalent metal ions which are useful in ion-exchange chromatography. The reference does not disclose the water-soluble polymeric chelates of the present invention which are useful to remove noxious NO from fluid streams.
U.S. Pat. Nos. 3,984,522 and 4,044,101, assigned to Asahi, disclose a process for removing NO from a gas stream the use of a solution containing a monomeric iron chelate and sulfite or sulfide. There is no disclosure regarding using a polymeric chelate of a metal, with separation of water and low molecular weight products and materials, from the chelate.
U.S. Pat. Nos. 4,079,118 and 4,091,074, assigned to Chisso Engineering Company, Ltd., disclose a process for the absorption of NO in a solution of ferric (or ferrous ion-ethylenediamine tetraacetic acid (EDTA) and sulfite. The liquid is treated to recover SO.sub.2, then by a complex procedure recovers iron by alkaline precipitation with recycle of the iron and EDTA and separation of the dithionate and other products for disposal. In this process, monomeric EDTA is lost by the system.
In U.S. Pat. Nos. 3,991,161; 3,992,508; 4,055,623 and 4,087,372, assigned to Kureha Kagaku Kogyo KK, are disclosed processes for absorbing NO in sulfite solutions containing iron salts to produce imidodisulfonates with subsequent hydrolysis to ammonium sulfate. There is no discussion of polymeric chelates or the problems encountered during the separation of excess water and buildup of products, by-products and materials.
Additional U.S. patents of general interest include U.S. Pat. Nos. 3,932,585; 4,081,509 and 4,126,529.
It is known by the number of techniques described above to treat a stack gas with a monomeric organic chelate of iron (II). A problem with these processes is that the efficient separation of excess water, products which build up during the reaction and byproducts from the monomeric organic chelate of iron (II) from the aqueous stream is usually difficult and very costly. A portion of the expensive iron-monomeric organic chelate is lost during separation of the low molecular weight materials, and cannot be recovered. It is therefore highly desirable to have a process where the expensive organic polymeric iron (III) or iron (II) chelate is separated easily from a waste stream originally containing NO, where the organic polymeric chelate of iron is recycled and used again and again.